Associate File Extension with Application

Solution 1:

The answer was a lot simpler than I expected. Windows Explorer has its own override for the open with application, and I was trying to modify it in the last lines of code. If you just delete the Explorer override, then the file association will work.

I also told explorer that I had changed a file association by calling the unmanaged function SHChangeNotify() using P/Invoke

public static void SetAssociation(string Extension, string KeyName, string OpenWith, string FileDescription)
{
    // The stuff that was above here is basically the same

    // Delete the key instead of trying to change it
    var CurrentUser = Registry.CurrentUser.OpenSubKey("Software\\Microsoft\\Windows\\CurrentVersion\\Explorer\\FileExts\\" + Extension, true);
    CurrentUser.DeleteSubKey("UserChoice", false);
    CurrentUser.Close();

    // Tell explorer the file association has been changed
    SHChangeNotify(0x08000000, 0x0000, IntPtr.Zero, IntPtr.Zero);
}

[DllImport("shell32.dll", CharSet = CharSet.Auto, SetLastError = true)]
public static extern void SHChangeNotify(uint wEventId, uint uFlags, IntPtr dwItem1, IntPtr dwItem2);

Solution 2:

Here's a complete example:

public class FileAssociation
{
    public string Extension { get; set; }
    public string ProgId { get; set; }
    public string FileTypeDescription { get; set; }
    public string ExecutableFilePath { get; set; }
}

public class FileAssociations
{
    // needed so that Explorer windows get refreshed after the registry is updated
    [System.Runtime.InteropServices.DllImport("Shell32.dll")]
    private static extern int SHChangeNotify(int eventId, int flags, IntPtr item1, IntPtr item2);

    private const int SHCNE_ASSOCCHANGED = 0x8000000;
    private const int SHCNF_FLUSH = 0x1000;

    public static void EnsureAssociationsSet()
    {
        var filePath = Process.GetCurrentProcess().MainModule.FileName;
        EnsureAssociationsSet(
            new FileAssociation
            {
                Extension = ".ucs",
                ProgId = "UCS_Editor_File",
                FileTypeDescription = "UCS File",
                ExecutableFilePath = filePath
            });
    }

    public static void EnsureAssociationsSet(params FileAssociation[] associations)
    {
        bool madeChanges = false;
        foreach (var association in associations)
        {
            madeChanges |= SetAssociation(
                association.Extension,
                association.ProgId,
                association.FileTypeDescription,
                association.ExecutableFilePath);
        }

        if (madeChanges)
        {
            SHChangeNotify(SHCNE_ASSOCCHANGED, SHCNF_FLUSH, IntPtr.Zero, IntPtr.Zero);
        }
    }

    public static bool SetAssociation(string extension, string progId, string fileTypeDescription, string applicationFilePath)
    {
        bool madeChanges = false;
        madeChanges |= SetKeyDefaultValue(@"Software\Classes\" + extension, progId);
        madeChanges |= SetKeyDefaultValue(@"Software\Classes\" + progId, fileTypeDescription);
        madeChanges |= SetKeyDefaultValue($@"Software\Classes\{progId}\shell\open\command", "\"" + applicationFilePath + "\" \"%1\"");
        return madeChanges;
    }

    private static bool SetKeyDefaultValue(string keyPath, string value)
    {
        using (var key = Registry.CurrentUser.CreateSubKey(keyPath))
        {
            if (key.GetValue(null) as string != value)
            {
                key.SetValue(null, value);
                return true;
            }
        }

        return false;
    }